Microwave components employed in antenna feed horns limit the operational bandwidth of reflector-based antenna systems, which typically require relatively wide band feeds in order to provide spectral coverage for non-contiguous satellite communication bands. When a single broadband device is used to provide coverage for both transmit and receive sub-bands, it is necessary that the combined bandwidth of the two sub-bands be very wide. For example, in the commercial C-band and Ku-band, as well as the military Ka-band, the ratio of the receive band frequencies to the transmit band frequencies is typically two to three (a forty percent bandwidth). On the other hand, the total transmit and receive bandwidth of the military X-band is relatively narrow at twelve percent, while the total transmit and receive bandwidth of the Extremely High Frequency (EHF) band comprising K and Q bands is considerably wider (at eighty-one percent).
When the transmit and receive bands are too widely separated (as in the case of the EHF band) it is necessary to use a dual horn feed (one feed per band) The problem can become complicated where available deployment space is constrained (such as in a shipborne application), mandating the use of a very compact single feed. In addition, as broadband demand continues to increase, it can be expected that satellites and associated earth terminals will have to operate over increasingly wider bandwidths.
When designing an antenna system that is to be capable of operating simultaneously over multiple bands (such as Ka band and X band, as a non-limiting example), with each band having its own pair of transmit and receive frequency bands, there may be a requirement for a composite feed having separate waveguide ports for each band and configured in a compactly nested architecture--something that is not provided by conventional waveguide horn designs.
Present day multiband feed architectures are typically either multiple feed systems employing frequency selective surfaces, or collocated/coaxial feeds with multiple ports for multiple bands. Because of its complexity, size and lengthy waveguides, the former approach cannot be used for a compact reflector system (such as a ring focus architecture) having a small aperture and small focal length to dish diameter ratio. The latter scheme has been implemented utilizing a nested coaxial feed approach, such as the dual band EHF feed (20 GHz-receive, 44 GHz-transmit) described in the U.S. Patents to Lee, U.S. Pat. No. 4,558,290; and subsequent Patents to Smith et al, U.S. Pat. No. 5,003,321; and Anderson et al, U.S. Pat. Nos. 5,635,944, 5,793,334, 5,793,335, 5,818,396 and 5,907,309.
For further examples of various coaxially positioned combinations of feed horn and cavity arrangements, attention may be directed to the U.S. Patents to Wilkes, U.S. Pat. Nos. 4,819,005 and 4,821,046; West, U.S. Pat. No. 5,216,432 and Weinstein et al, U.S. Pat. No. 5,635,944. Additional illustrations of (non-coaxial) multiband feeds include those described the U.S. Patents to Frosch, U.S. Pat. No. 4,258,366; Luly, U.S. Pat. No. 4,801,945; Gauthier et al, U.S. Pat. No. 4,847,574; and Smith, U.S. Pat. No. 5,258,768.
In the coaxial/nested approach to the dual band feed, where each band is broadband by reason of having separate transmit and receive bands, the problem of effective launching, transmission and radiation had previously only been successfully solved by employing a turnstile launching mechanism. In this approach, a pair of orthogonally polarized ports are employed, each with a pair of oppositely positioned launching ports, with a total of four launching ports--thus, the name turnstile. This has been the only effective way to forcibly balance the excitations and launch the coaxial TE.sub.11 mode avoiding the fundamental TEM mode and the higher order modes. The turnstile mechanism with external waveguides T's and phase shifters have also provided for a polarizer to generate circular polarization.
A major shortcoming of a turnstile configured approach is the significant size, weight and complexity of its associated waveguide `plumbing`. In order to effectively eliminate such plumbing, first, it is necessary to provide some form of design having a pair of singularly launched ports, one for each on of the two orthogonal polarizations, into a coaxial waveguide without exciting higher order modes or the fundamental TEM mode. Secondly, it requires a novel design of a broadband polarizer implemented internally (rather than externally) of the cylindrical waveguide. As will be described below, the present invention successfully accomplishes these objectives.
In addition to collimating a beam, it is customarily required that the antenna system be capable of tracking its associated satellite, as this not only ensures an uninterrupted link, but also assists in initial acquisition of the satellite. For this purpose, it is customary practice to either use a difference pattern (if available) in the antenna's directivity profile, or physically dither the main beam about the link axis which can be very difficult in the case of a platform that is dynamic and/or has significant inertia. Other forms of tracking on the main beam include sequential lobing and nutating feeds, which have a higher error slope at the expense of beam offset loss. The use of a difference pattern is preferable as it can provide an error-slope for a very accurate and rapid response tracking scheme, and can be used in both monopulse and pseudomonopulse systems.
In a multiband system that does not employ co-located feeds, but instead has a dual reflector architecture with a frequency selective surface to partition its aperture into real and virtual focal points, a pointing error between the two feeds may occur. When one of the bands has a much high frequency band, it may be necessary to track at the higher frequency band, and rely on the broader beam coverage of the lower frequency band to avoid a pointing loss. As the band of operation becomes higher, as in the case of fixed size, Ka-band reflector systems, for example, the antenna beamwidth becomes very narrow, so that using the main beam for tracking introduces the issues of tracking stability and speed.
For examples of literature describing various types of tracking feeds, attention may be directed to the U.S. Patents to Thomas, U.S. Pat. No. 4,849,761 and 5,036,332, and an article by P. Patel, entitled: "Design of an Inexpensive Multi-Mode Satellite Tracking Feed," IEEE Proceedings, 1988.
Further, for examples of literature describing what may referred to as `compensating` type polarizer structures, that employ one or more sets of vanes or fins and pins configured as conductive or dielectric elements, attention may be directed to the U.S. Patents to DiTullio, U.S. Pat. No. 4,100,514 and Saad, U.S. Pat. No. 4,672,334.